constant current diode driver (easy!)

[glowdye]

I have copied a schematic from the book " art of electronics" by Horowitz. I believe i have found a very easy to do laser diode driver. Its from page 76.
Current = Vbe/R2 VBE is found on the datasheet of your pnp pnp pair. Vbe is usually .65 volts
load is the laser diode connections.

Do your math and scale up your transistors depending on how much current you need to feed these guys. use fractional ohm resistors to get multiamp currents.
a good rule of thumb is small currents <100ma use to-92 >500ma use tipc41+tipc42 for >5A use 2n3055 transistors. One big problem is that this constant current regulator is not temperature stabilized according to the text the current decreases .3% for every .C increase in temperature.

 

[phenol]

this way of limiting the current using the drop across a shunt resistor to bias a bipolar transistor is commonly used in power supplies.
you can use npn transistors for a current sink solution

 

[norbyx]

So what's the down side?

 

[phenol]

stability and efficiency. there is one Vbe drop across the shunt resistor

 

[Cyparagon]

There are 2 downsides I can see. You need to re-design the circuit for different voltages/currents, and there are more parts than an lm317.

 

[Meatball]

Running a transistor in the linear region causing it to absorb a lots of power fairly quickly. I do not think this is a good circuit to scale up with.

 

[Cyparagon]

Uh... only because it's dropping the extra voltage. That's what linear drivers are supposed to do.